1. Field of the Invention
This invention relates to optical waveguides and a process for making them.
2. Description of Related Art
Much work has been directed toward developing optical communications systems. The basic element of such systems is the optical waveguide which transmits or guides optical waves from one point to another and serves to connect various devices in an optical integrated circuit. Such waveguides consist of an optically transparent waveguiding medium surrounded by media with lower indices of refraction so that light propagating along the waveguide is totally reflected at the boundaries with the surrounding media. In optical integrated circuits, the two basic waveguide geometries are the planar or slab waveguide and the channel or strip waveguide.
U.S. Pat. No. 3,998,687, issued to Ballman, et al., on Dec. 21, 1976, discloses a technique for growing an epitaxial thin film of lithium niobate involving immersing a lithium tantalate substrate seed crystal into a supercooled melt of a flux system selected from among Li.sub.2 B.sub.2 O.sub.4 -Li.sub.2 Nb.sub.2 O.sub.6, Li.sub.2 WO.sub.4 -Li.sub.2 Nb.sub.2 O.sub.6, K.sub.2 WO.sub.4 -Li.sub.2 Nb.sub.2 O.sub.6, and WO.sub.3 -Li.sub.2 Nb.sub.2 O.sub.6, and after establishing equilibrium, withdrawing the seed from the flux and cooling the withdrawn seed. The films are disclosed to be useful as optical waveguides.
U.S. Pat. No. 4,073,675, issued to Ballman, et al., on Feb. 14, 1978, discloses a process for producing epitaxial crystalline films of LiNbO.sub.3 on LiTaO.sub.3 substrates on a selected crystallographic plane comprising applying a powder of LiNbO.sub.3 on the LiTaO.sub.3 substrate, heating to a temperature above the melting temperature of LiNbO.sub.3 but below the melting temperature of LiTaO.sub.3, and then cooling slowly, i.e., between 10.degree. and 50.degree. C. per hour to below the melting point of LiNbO.sub.3.
U.S. Pat. No. 4,037,005, issued to Phillips on July 19, 1977, discloses a method for making optical waveguides wherein a niobium or niobium-oxide coated single crystal of lithium tantalate is heated to allow niobium to diffuse into the crystal and then the crystal is cooled. When the crystal is coated with niobium, the niobium is oxidized during the lower temperature range in the heating step. Diffusion occurs at about 1050.degree. to 1250.degree. C.
U.S. Pat. No. 4,284,663, issued to Carruthers, et al., on Aug. 18, 1981, discloses a method of fabricating an optical waveguiding surface layer in an optically transparent crystalline substrate, such as lithium niobate, by depositing a layer of a metal selected from transition element metals having an atomic number of 21 to 30, as well as silver and gold, onto a surface of the substrate and heating the resultant product at about 800.degree. C. to 1100.degree. C. to indiffuse the metal.
U.S. Pat. No. 4,206,251, issued to Chen on June 3, 1980, and U.S. Pat. No. 4,329,016, issued to Chen on May 11, 1982, disclose a process for diffusing a metal into a substrate which may be either a semiconductor material or a dielectric material comprising coating the substrate with a liquid composition comprising organo-metallic solutions of the desired metal and silica, heating the resulting coated substrate at an elevated temperature for a period of time sufficient to cause the organic portion of the solution to decompose, thereby leaving a composite film comprising an oxide of the desired metal and silica, heating the resulting structure further to cause diffusion of the metal from the metal oxide into the substrate. LiNbO.sub.3 is specifically mentioned as a substrate and diffusion of Ti is specifically discussed. The patents also disclose the optical waveguide device formed by the process. The composite film formed in the process prevents Li.sub.2 O out-diffusion.
Ishitani, et al, Appl. Phys. Lett 29, 289-291 (1976), disclose an optical waveguide comprising a single-crystal Sr.sub.2 Nb.sub.2 O.sub.7 film grown onto a b-plate Sr.sub.2 Ta.sub.2 O.sub.7 single-crystal substrate by rf sputtering.
U.S. Pat. No. 3,949,323, issued to Bierlein, et al., on Apr. 6, 1976, discloses nonlinear optical devices and electro-optic modulators which use a crystal consisting of a compound having the formula MTiO(XO.sub.4), wherein M is at least one of K, Rb, Tl or NH.sub.4 and X is at least one of P or As and wherein X is P when M is NH.sub.4.
Zumsteg, et al., Journal of Applied Physics 47, 4980-4985 (1976), disclose that K.sub.x Rb.sub.1-x TiOPO.sub.4 is a nonlinear optical (NLO) material with large NLO coefficients, transparent over a large range of wavelengths, not readily susceptible to laser damage, and chemically inert.
U.S. Pat. No. 4,231,838, issued to Gier on Nov. 4, 1980, discloses a process for the manufacture of single crystals of MTiOXO.sub.4, wherein M is K, Rb or Tl and X is P or As, of optical quality and of sufficient size for use in nonlinear optical devices, said process comprising heating starting ingredients, chosen to be within the region of the ternary phase diagram in which the desired crystal MTiOXO.sub.4 product is the only stable solid phase, to produce MTiOXO.sub.4, and then controllably cooling to crystallize the desired product. Crystals which have mixtures of elements for M and/or X can be grown by the process.
U.S. Pat. No. 4,305,778, issued to Gier on Dec. 15, 1981, discloses a hydrothermal process for growing single crystals of MTiOXO.sub.4, wherein M is K or Rb and X is P or As, said process involving using as a mineralizing solution an aqueous solution of a glass defined by specified portions of the ternary diagrams for the selected M.sub.2 O/X.sub.2 O.sub.5 /(TiO.sub.2).sub.2 system.
Publications discussing the nonlinear or electro-optic properties of KTiOPO.sub.4 (KTP) include Belt, et al., Laser Focus/Electro-optics, 110-124 (Oct. 1985), and Massey, et al., Applied Optics 24, 4136-4137 (1980).